Why are animal crossing furniture items displayed as leaves
Solution 1:
Leaves transforming into objects is a cultural reference in Japan. Without going into too much detail, it's a reference to the mythological ability of the tanooki/tanuki (the species which Tom Nook is actually a member of), which supposedly could use leaves to help them transform. This also isn't unique to the tanuki, the kitsune also demonstrates this ability (and possibly other creatures as well).